1. Field of the Invention
The present invention relates to novel Group IVA metal-rich, crystalline molecular sieve compositions and their method of preparation and their use as ion exchange materials. These compositions are prepared hydrothermally from gels containing reactive Group IVA metals, silicon and phosphorous compounds and alkali metal cations.
2. Description of the Previously Published Art
Molecular sieves of the crystalline aluminosilicate type, usually referred to as zeolites, are well known in the art and now comprise a large number of species of both naturally occurring and synthetic compositions. In general these crystalline zeolites are formed by corner sharing ALO.sub.2 and SiO.sub.2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without displacing any atoms which make up the permanent crystal structure.
In recent years, crystalline aluminophosphate compositions have been reported which are microporous framework oxide molecular sieves. These materials are formed from AlO.sub.2 and PO.sub.2 corner-shared tetrahedra, and their intracrystalline pore volumes and pore diameters are comparable to those known for the zeolites. These materials need not, however, have ion-exchange capacity.
The term "molecular sieve" refers to a material having a fixed, open network structure capable of reversibly desorbing an adsorbed phase, and that may be used to separate hydrocarbons or other mixtures by selective occlusion of one or more of the constituents. The network structure, however, does not necessarily need to be composed of vertex-shared tetrahedra as in the zeolites or the aforementioned aluminophosphates. Molecular sieves, both zeolitic and non-zeolitic, are known to have catalytic capabilities for various hydrocarbon conversion reactions such as alkylation, cracking, hydrocracking, isomerization, etc. Some also have capabilities for the conversion of nitrogen oxides as practiced in the art of exhaust pollution control. In addition, molecular sieves can exhibit capabilities as ion-exchange materials.
Substantial success in preparing titanium-containing zeolites and molecular sieve compositions have been reported by Lok et al in U.S. Pat. No. 4,707,345, EP 0161488, EP 0158349, EP 0158350, EP 0158977, WO 85/04853, WO 85/04856, and Taramasso et al in U.S. Pat. No. 4,410,501. These cited compositions involve the incorporation of titanium into silicate, aluminosilicate, phosphoaluminate and phosphoaluminosilicate frameworks composed of vertex shared tetrahedra. The compositions are microporous, crystalline zeolites and molecular sieves formed from corner sharing PO.sub.2, AlO.sub.2 and SiO.sub.2 tetrahedra. In all of these patents the titanium is described as being present in the materials as the tetrahedral TiO.sub.2 oxide. Furthermore, the titanium oxide contents of these cited materials are generally low, usually less than 10 wt % of the bulk material on an anhydrous basis. In the present invention, the novel Group IVA containing molecular sieves are structurally and chemically distinct from the above cited compositions as seen by comparison of the x-ray powder patterns and chemical compositions of the molecular sieve phases. Furthermore, the present Group IVA containing molecular sieves are composed of different framework structural units than the zeolites and molecular sieves familiar in the art, that is, they are composed of edge shared octahedra connected through their vertices to tetrahedra.
There are other known compositions of molecular sieve materials which are not composed solely of interconnected oxide tetrahedra, but which are composed of octahedral and tetrahedral framework constituents. A relevant example is the family of materials, discussed by J. Zeeman (Acta Cryst, 12, 1959, p. 252) that exhibit a molecular sieve structure in which the framework constituents have coordination numbers of both six and four. These materials are substantially similar structurally to the novel titanium containing molecular sieves described herein, but have distinctly different chemical compositions. The term "substantially similar" means that the relative arrangement of tetrahedral and octahedral units in the framework is described by the reported structure, but does not restrict the framework of the present invention to identical unit cell parameters, x-ray diffraction d-spacings or line intensities, or extra-framework atom locations. The reported compositions observed for these materials contain germanium (Nowotny et al, Montash., 85, 1954, p. 558), iron and arsenic as in the mineral pharmacosiderite (Hagele, G. and Machatschki, F Fortschr d. Mineralogie, 21, 1937, p. 77) and aluminum and arsenic as in the mineral aluminopharmacosiderite (Zeeman, Acta Cryst. 12, 1959, p. 252). There are no previously reported examples of this structure that contain Group IVA metals as a framework constituent.
3. Objects of the Invention
It is an object of this invention to obtain a unique class of Group IVA metal silicates, Group IVA metal phosphates and Group IVA metal phosphosilicates which are microporous.
It is further object of this invention to obtain a unique class of Group IVA metal silicates, Group IVA metal phosphates and Group IVA metal phosphosilicates which have characteristics of both the aluminosilicate zeolites and the aluminosphosphate molecular sieves.
It is further object of this invention to obtain a unique class of Group IVA metal silicates, Group IVA metal phosphates and Group IVA metal phosphosilicates having a three-dimensional microporous crystal framework structure of tetrahedral and octahedral units.
It is further object of this invention to provide a process for the preparation of crystalline, microporous compositions containing Group IVA metals.
It is further object of this invention to provide crystalline, microporous compositions which possess activity for hydrocarbon conversions, exhaust effluent conversions, and which may be used as ion exchange media or adsorbents.
These and still further objects of the present invention will become readily apparent to one skilled in the art from the following detailed description and specific examples.